By all means, start stockpiling parts (in good condition), as 5 speed parts are no longer available. Have a good look at ebay as people try and offload their bikes like this for whatever money they can get, otherwise it goes to the tip. 8 and 9 speed parts will not fit your bike due to the difference in axle and hub widths to fit the extra gears. Perhaps invest in some new gear and brake cables to make shifting and braking smooth, and just keep the bike properly adjusted. Don't worry about the cosmetic appearance of the bike as this will only attract the undesirables.Enjoy the bike.

Six speed will fit but the small cog will foul the frame and the derailleur will foul the spoke guard on the large cog.

Answer? Place a washer on the axle to move the frame away from the rear cog and remove the spoke guard. The derailleur and shifters will still cover the full range - at least, that's how it worked on the lad's seventies Gitane - it'll be a suck it and see effort with any bike.